EAS systems are typically used to prevent the unauthorized removal of articles or merchandise from public places such as stores or libraries. Articles that are frequently subject to theft include books, audiotapes, videotapes, and compact discs. EAS systems generally use EAS markers, which can be selectively sensitized or desensitized, that are placed on articles desired to be protected. The markers work in combination with detectors typically located at the exits of the area containing the marked articles. When a particular marker is desensitized, it can be moved past the marker detector without activating an alarm. When the marker is sensitized, the marker will trigger an alarm when it is moved past the marker detector. EAS markers are commonly desensitized by desensitizing machines that are under the exclusive control of authorized personnel.
One common type of marker detector uses an alternating magnetic interrogation field that is generated at the exits of the area protected by an EAS system. A marker used in association with such a detector typically includes an elongated strip of low-coercivity magnetic material in which the domains of the low-coercivity magnetic material are alternately switched in polarity from north to south and back. The low coercivity magnetic material disturbs the magnetic interrogation field in a detectable manner, which triggers an alarm. The alarm indicates that the article carrying the marker has not been properly checked out.
Throughout the specification, the terms desensitize and deactivate have been used interchangeably with respect to surveillance markers. Generally, such terms are intended to mean that a given surveillance marker has been transformed to a state where the marker will not trigger or set off an EAS detector. One particular type of marker suitable for use with the present invention includes an elongated strip of low-coercivity magnetic material that is divided into multiple sections by at least one magnetizable section (referred to as a "keeper") having a higher magnetic coercivity. Such a marker is preferably desensitized or deactivated by magnetizing the keepers of the marker. When the keepers are magnetized, the marker will not be detected when placed in an alternating magnetic field interrogation zone generated by an EAS detector. When the keepers are demagnetized, thus activating the marker, the marker will be detected. One example of a such a marker is available from Minnesota Mining and Manufacturing Company of St. Paul, Minn. (3M) under the designation DSB-2, which is sold under the name TATTLETAPE.TM.. This marker is described in U.S. Pat. No. 3,747,086, which is incorporated by reference herein.
Although the prior art provides magnets for desensitizing markers on articles, there are some limitations to the conventional approaches. First, some of these approaches allow for the marker to pass the desensitizing magnet while the magnet is not optimally oriented for desensitizing the marker. For example, the magnetic field profile to which the marker is exposed may be asymmetrical, or may present an odd number of magnetic transitions with regards to this marker. As a result, the marker may be inadvertently resensitized after being desensitized, and thus trigger an alarm when none is intended. Similarly, the prior art approaches may fail to desensitize the marker at all if the magnet is not properly aligned with the marker, or the entire marker is not moved past the magnet. In any of these situations, a marker can then trigger an alarm when a patron attempts to exit the library.
The present invention provides solutions to these and other problems, and offers other advantages over the prior art designs.